1. Field of the Invention
The present invention relates to an image processing apparatus for processing a tomographic image of an eye to be examined and a method thereof.
2. Description of the Related Art
A tomography imaging apparatus such as an OCT (Optical Coherence Tomography) for an eye can three-dimensionally observe the internal state of a retinal layer. The tomography imaging apparatus has recently received a great deal of attention because of its usefulness in more properly diagnosing diseases. An example of the form of OCT is a TD-OCT (Time Domain OCT) that combines a wideband light source and a Michelson interferometer. The TD-OCT is configured to scan the delay of a reference arm to measure the interference light with the backscattered light of a signal arm, thereby obtaining information of depth resolution. However, it is difficult to quickly acquire an image by the TD-OCT. As a method of acquiring an image at a higher speed, an SD-OCT (Spectral Domain OCT) is known, which uses a wideband light source and acquires an interferogram by a spectroscope. Also known is an SS-OCT (Swept Source OCT) that employs a technique of measuring spectral interference by a single-channel photodetector using a high-speed wavelength swept light source as a light source. In addition, as a functional OCT, a PS-OCT (Polarization Sensitive OCT) is known, which performs imaging using a polarization parameter (retardation and orientation) as one optical characteristic of the tissues of the fundus of an eye.
If a shape change of a retina can be measured in tomographic images captured by the above-described OCTs, the degree of progression of a disease such as glaucoma, age-rated macular degeneration, or diabetic macular edema and the degree of recovery after treatment can quantitatively be diagnosed. To quantitatively measure the shape change of a retina, Japanese Patent Laid-Open No. 2011-030626 (to be referred to as literature 1 hereinafter) discloses a technique of analyzing the degree of unevenness of a retinal layer in age-rated macular degeneration using a computer.
In literature 1, the difference in thickness between a retinal layer obtained by analysis and an estimated normal retinal layer is evaluated. In diabetic macular edema, when a retina swells due to edema, the eyesight gets worse. Hence, treatment is given to decrease the retinal thickness, and the effect of treatment is confirmed using the OCT. In fact, a distortion in the layer structure of the retinal layer, particularly, a change in a photoreceptor layer affects visual performance more than the retinal thickness. Conventionally, a person observes images and judges the degree of distortion in a retinal layer based on his/her experiences. This poses a problem from the viewpoint of objectivity.